There have been proposed a plurality of single and multiple layer sheet structures for packaging of products requiring significant functional protection from transmission of gaseous materials into or out of the package through the packaging film. For example, various food type products require barriers to transmission of oxygen, moisture, oils and the like. Numerous conventional single and multiple layer structures have been made to serve this end, with a moderate degree of success. Such structures include, for example, saran coated polypropylene, glassine, and the like. It is known to use multiple layer structures into which may be incorporated a separately formed layer of oriented polypropylene or oriented polyester, as well as functional barrier layers.
While such multiple layer films incorporating an oriented layer do provide certain improvements, they require a multiplicity of manufacturing steps which puts them at a competitive cost disadvantage. Advantageously, though, the functional barrier benefits of the multiplicity of layers does provide overall improved barrier functions.
Other desirable attributes of film packages, and particularly those used for packaging snacks, such as potato chips or corn chips, are related to the appearance and feel of the package; which translates into consumer perception of the package at the point of sale. Particularly desirable perceived attributes are gloss and stiffness.
A highly desirable combination of attributes, then includes functional barrier properties to certain gaseous transmission, and perceived properties of gloss and stiffness.
Various attempts have been made to advantageously utilize the benefits of molecular orientation of films to achieve certain of the desirable properties. A serious problem in these developments has been that each different polymer has its unique required set of heating and stretching conditions. Where certain combinations of layer compositions do not have overlapping conditions conductive to molecular orientation of the multiple layer film, additional provisions must be made for effecting the simultaneous orientation of the plurality of layers. There is no known art which makes the necessary provisions. Absent these provisions, in previous attempts to orient multiple layer films, adjacent layers have developed undesirable stresses at layer interfaces, and cohesive stresses within the layers themselves. These stresses too often have manifested themselves in poor or non-existent layer adhesion at the layer interfaces, and in cracking or hole development in one or more of the layers.
Mueller, U.S. Pat. No. 4,188,443 handles this problem in a 5 layer film by selecting the compositions of the second and fourth layers such that they are above their melt temperature during the orientation process (col. 5 line 43). While this mechanism is successful in relieving the interfacial stresses of the orientation process, only 3 of the 5 layers are truly molecularly oriented, and selection of material compositions for layers 2 and 4 may be severely limited by the melting temperature requirements.
Yamada, U.S. Pat. No. 4,261,473 teaches a balanced 3 layer film, as in his EXAMPLE 10, wherein the outer layers are polyethylene terephthalate and the core layer is EVOH. Sheets of this film are preheated for a lengthy 5 minutes, apparently to reach steady state temperature throughout the film thickness, before the film is stretched by drawing it into a "cup" shaped mold. Indeed, this process is more closely related to conventional thermoforming than to molecular orientation.
Mueller, U.S. Pat. No. 4,194,039 teaches a "balanced" 3-layer film (col. 6 line 23) that is a combination of olefins and olefin blends. The film is made by a plurality of extrusion steps and orientation steps.
Bornstein U.S. Pat. No. 4,064,296 teaches an oriented 3 layer film having EVOH as the core layer. However, in Bornstein's film it is "crucial" (col. 4 line 65) that one of the two outer layers be cross-linked, i.e. by irradation.
It is an object of this invention to provide a multiple layer packaging film material which is economically competitive to make and which has a combination of attributes including, as functional physical properties, high barrier to gaseous transmission through the film, and as perceived properties, high gloss, transparency, and stiffness. Preferrably the films are unbalanced in that conventional heat sealing equipment may be used to apply heat to the more heat stable side of the film, driving the heat through the film without undue distortion of the film to effect heat sealing of the layer on the opposite surface of the film. Such unbalanced films are readily adapted for use in conventional packaging equipment to economically package a variety of products.